Lacustrine sedimentological and geochemical records for the last 170 years of climate and environmental changes in southeastern China

Reconstruction of modern climate and environmental changes in east Asia using inland natural climate archives can provide valuable insights on decadal–multidecadal climate and environmental patterns that are probably related to both natural and anthropogenic forcing. Here we investigated an 89‐cm‐long sediment core (TH1) from Tian Lake, southeastern China, for sedimentological, physical and geochemical parameters in order to understand climate and environmental changes for the latest two centuries. ¹³⁷Cs‐ and ²¹⁰Pb‐based age models show that the fine sand–coarse silt‐dominated core contains ~170 years (c. AD 1842–2011) of continuous sedimentation. Sediments with fine sands, low MS values, high water content, high TOC content and a high C:N ratio from c. AD 1842 to 1897 suggest intense hydrological conditions and strong runoff in the catchment, probably because of a humid climate. From AD 1897 to 1990, sediments with very fine sand and coarse silt, high MS values, low water content and unchanged TOC and C:N ratios indicate normal hydrological conditions and in‐lake algae‐derived organic matter. During this interval, the chemical weathering indicators show stronger weathering conditions compared with sediments deposited during AD 1842–1897, supporting the dominance of weathered surface soil input in the earlier interval and physical erosion dominance in the later period, respectively. Since AD 1990, the continuous decrease of geochemical proxies suggests human‐interacted Earth surface processes in the catchment of Tian Lake. A PCA revealed four dominant geochemical controlling factors – detrital input, trophic status, grain size and early diagenesis –, accounting for 26, 20, 18 and 16% of total variance, respectively. This study for the first time provides lacustrine geochemical evidence for the most recent two centuries of climate and environmental changes in coastal southeastern China, a region that is currently undergoing an inversion of critical zone, i.e. an overturning of its soil profile, owing to swift modernization.     

Hydrochemistry,origin and evolution of sedimentary subsurface fluids: I. Early diagenetic pore-water evolution in low-sedimentation rate offshore basins

Pore water studies enable (1) the detection of diagenetic reactions actively occurring in the sediment at the time of sampling,(2) the distinction between principal modes of solute transport, i.e., between advection (convection) and diffusion, and (3) the assessment of mineral-solution equilibria, Pore waters are, therefore, preferred diagnostic objects in the study of diagenesis, particularly early diagenesis. The single most important factor for pore-fluid evolution in modern offshore basins is sedimentation rate which is closely correlated with organtic matter content of the sediment. Organic matter represents the most reactive sediment constituent which, through bacterial decomposition, provides some of the main solutes involved in early diagenetic mineralization reactions. On the basis of sedimentation rate and organic matter content, it is convenient to distinguish two end-members of basins (environments) with respect to early diagenesis: I. Low to intermediate-sedimentation rate basins with convection or diffusion-controlled pore-water profiles and II. high-sedimentation rate basins with reaction-controlled pore-water profiles. The first group of basins or environments, which is the subject of this paper, is typically represented by pelagic sediments. Three principally different trends of pore-water evolution occur: 1. convection-controlled profiles with no pronounced vertical gradients for the dissolved species; 2. diffusion-controlled profiles with vertical gradients but linear correlations between major ions, especially Ca and Mg, and 3. profiles with gradients but no linear correlation between the major ions. The later are transitional to the trends seen in high-sedimentation rate basins. With respect to redox-potentials, the diagenetic environments of low-sedimentation rate basins are generally oxic to suboxic.     

Hydrothermally induced diagenesis: Evidence from shallow marine-deltaic sediments,Wilhelmøya,Svalbard

Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelm?ya(Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent(～12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance(more than five times the thickness of the sill)from the intrusions. We present evidence that the sedimentary sequence can be divided into two units:(1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃,and(2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures(around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ～40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization(illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals.     

Isotopic signature of burial diagenesis and primary lithological contrasts in periplatform carbonates (Miocene, Great Bahama Bank)

The stable isotope geochemistry of Miocene sediments from the leeward margin of the Great Bahama Bank was examined to investigate burial diagenetic processes in periplatform carbonates. Data indicate that, in addition to differences in bulk proportions of neritic and pelagic carbonate along the slope, rhythmic variation in primary carbonate content has controlled patterns of burial diagenesis and associated geochemical signatures throughout much of the succession examined. The present study focuses on Ocean Drilling Program Sites 1006 and 1007, the most distal of five sites drilled from marginal to deep basin environments during Leg 166. These Miocene sections are characterized by their cyclic appearance, manifest as decimetre‐ to metre‐scale alternations between light‐coloured ooze/chalk/limestone and dark‐coloured marl/marlstone. The section at Site 1006 contains a high proportion of pelagic carbonate and is unlithified to a subbottom depth of ～675 m. Fluctuations in δ¹⁸O and δ¹³C values at this site are independent of lithological variation and reflect primary conditions. At Site 1007, located at the toe‐of‐slope and composed of a mixture of bank‐derived and pelagic carbonate, limestones are densely cemented, show little evidence of compaction and have δ¹⁸O values up to 2‰ higher than coeval sediments at Site 1006. Marlstones at Site 1007 are poorly cemented, exhibit an increase in compaction‐related features with depth and have lower and more variable δ¹⁸O values that are similar to those of coeval sediments at Site 1006. Isotopic and petrographic characteristics of limestone interbeds result from cement precipitation from cold sea water during the first ～100 m of burial. Higher proportions of insoluble materials and pelagic carbonate seem to have inhibited diagenetic alteration in adjacent marlstones; in spite of significant compaction and pressure solution during burial, original isotopic compositions appear to be best preserved in these intervals at Site 1007. The documented contrasts in petrographic and isotopic patterns illustrate the role of primary sediment composition in controlling lithification processes in periplatform carbonates and stress the importance of considering such factors when interpreting geochemical data from ancient shelf and slope limestones.     

Lipid geochemistry of sediments from Upton Broad,a small productive lake

Major lipid classes isolated from 3 sections of the sediment profile of Upton Broad, a shallow lake formed about 500 yr ago, were correlated with input and with changes attributable to early diagenesis. Surficial sediment contains lipids characteristics of the known recent algal and microbial input; preservation of chlorophyll pigments and absence of appropriate markers suggest that diagenesis is minimal. Older deposits from the eutrophic phase of the lake's history contain higher levels of bacterial lipids and diagenetic products. The earliest sediment contains lipids characteristic of terrigenous input subjected to more extensive diagenesis than the younger sediment. Recognition of early diagenesis in anoxic sediment suggests that distributions of the shorter-chain homologous free n-alkanes, alkanols and alkanoic acids result from microbial synthesis using material initially deposited at the sediment surface, rather than partial preservation of the lipids of primary source organisms.     

Diagenetic cycling of trace elements in the bottom sediments of the Swan River Estuary,Western Australia

Teflon strips were used in-situ in the bottom sediments at two sites in the Swan River Estuary to collect diagenetic Fe–Mn oxyhydroxides and monitor monthly changes in their morphology and trace element geochemistry. This study demonstrates that substantial concentrations of trace elements accumulate at the redox front during the formation of diagenetic Fe–Mn oxyhydroxides. It is likely that the Fe–Mn oxyhydroxides initially nucleate and grow on the Teflon strips via bacterial activity. Trace element geochemistry of the diagenetic Fe–Mn oxyhydroxides is influenced by changes in the supply of trace elements from either the bottom sediments and/or water column or changes in the physico-chemical status of bottom and porewaters. If sufficient diagenetic Fe–Mn oxyhydroxides are preserved in the upper layer(s) of the bottom sediment it is possible that diagenetic (secondary) trace element enrichment profiles may be produced which modify the historical input of natural or anthropogenic trace element sources. Alternatively, partial or complete dissolution of the diagenetic Fe–Mn oxyhydroxides in response to temporal changes in the redox status of the bottom sediment may lead to a substantial underestimate of trace element fluxes in historical bottom sediment profiles. This study highlights that considerable care must be taken when interpreting short- to long-term geochemical profiles in bottom sediments due to the possible occurrence of rapid, seasonally mediated diagenetic processes.     

Holocene climate vs. catchment forcing on a shallow,eutrophic lake in eastern Poland

This paper presents a sedimentary record from Lake ?ukie located in the southeastern part of the Central European Plain, beyond the reach of the maximum extent of the last glaciation. The lake has thermokarstic origin and developed during the last glacial termination due to subsidence of the sub‐Quaternary carbonate basement triggered by permafrost thawing. A sediment core was investigated to reconstruct water trophic state and lake depth changes during the Holocene. We aimed at showing the relationship between ecological and geochemical changes in the lake and regional/supraregional climatic and hydrological trends throughout the Holocene. Results of subfossil Cladocera analysis were combined with data on the geochemistry and stable C and O isotopes in sedimentary carbonates. Isotopic and geochemical proxies helped to detect sources of sedimentary particles in the lake and thus to reconstruct changes in the intensity of atmospheric and catchment processes (e.g. precipitation and surface runoff). The Cladocera analysis results indicated endogenic processes in the lake such as trophic changes. Our data revealed that Lake ?ukie has always been a rather eutrophic water body and the periods of particularly high productivity were in the lower Preboreal and upper Subatlantic. Periods of increased water depth were recorded in the lower Preboreal, lower Boreal and upper Subboreal, whereas low water stands were obtained during the late Preboreal, late Boreal, late Atlantic and Atlantic/Subboreal transition as well as during the lower Subboreal. The sediment succession from Lake ?ukie provides the first full Holocene record of carbon and oxygen stable isotopes in lacustrine carbonates from the eastern part of the Central European Lowland. The record is characterized by uncommonly high δ¹³C and δ¹⁸O values of the carbonates resulting from a combination of within‐lake processes and dissolution of the carbonate bedrock of Cretaceous age. The impact of the old carbonates on isotope values was helpful in the reconstruction of the catchment forcing on the lake.     

Encounters with an unearthly mudstone: Understanding the first mudstone found on Mars

The Sheepbed mudstone forms the base of the strata examined by the Curiosity rover in Gale Crater on Mars, and is the first bona fide mudstone known on another planet. From images and associated data, this contribution proposes a holistic interpretation of depositional regime, diagenesis and burial history. A lake basin probably received sediment pulses from alluvial fans. Bed cross‐sections show millimetre to centimetre‐scale layering due to distal pulses of fluvial sediment injections (fine‐grained hyperpycnites), fall‐out from river plumes, and some aeolian supply. Diagenetic features include mineralized synaeresis cracks and millimetre‐scale nodules, as well as stratiform cementation. Clay minerals were initially considered due to in situ alteration, but bulk rock chemistry and mineralogy suggests that sediments were derived from variably weathered source rocks that probably contained pre‐existing clay minerals. X‐ray diffraction analyses show contrasting clay mineralogy in closely spaced samples, consistent with at least partial detrital supply of clay minerals. A significant (ca 30 wt%) amorphous component is consistent with little post‐depositional alteration. Theoretical modelling of diagenetic reactions, as well as kinetic considerations, suggest that the bulk of diagenetic clay mineral formation occurred comparatively late in diagenesis. Diagenetic features (synaeresis cracks and nodules) were previously thought to reflect early diagenetic gas formation, but an alternative scenario of synaeresis crack formation via fabric collapse of flocculated clays appears more likely. The observed diagenetic features, such as solid nodules, hollow nodules, matrix cement and ‘raised ridges’ (synaeresis cracks) can be explained with progressive alteration of olivine/glass in conjunction with centrifugal and counter diffusion of reactive species. Anhydrite‐filled fractures in the Sheepbed mudstone occurred late in diagenesis when fluid pressures built up to exceed lithostatic pressure. Generating fluid overpressure by burial to facilitate hydraulic fracturing suggests a burial depth of at least 1000 m for the underlying strata that supplied these fluids.     

北黄海沉积物中氮的地球化学特征及其早期成岩作用

本文首次报道对北黄海沉积物中氮的形态及其早期成岩作用的研究成果。结果表明 ,北黄海沉积物中占主导地位的是细粒度组分 ,其结构和性质直接影响着沉积物中化学元素的形态、含量与分布 ,使自然粒度沉积物中各形态氮的垂向分布与细粒度组分中各形态氮的分布非常相近。在氮的早期成岩过程中 ,粒度也起着非常重要的作用 ,沉积物的粒度越细 ,沉积物中有机氮的分解速率越小 ,即细粒度沉积物中有机氮的分解矿化速率最小 ,易于富集有机氮。对于不同站位的沉积物来说 ,因其沉积速率的差异 ,致使沉积物中各形态氮的埋藏通量有很大差异 ,沉积速率越大 ,沉积物中各形态氮的埋藏通量越大 ,在沉积速率最大的 C4站位 ,各形态氮的埋藏通量最大。     

长江口及其邻近陆架区沉积物的早期成岩作用

通过水下浅层及近岸钻探取样,对样品的pH值与Eh值,有机碳、细菌、孔隙水化学组份、自生矿物标型特征以及碳氧同位素等的分析。研究了沉积物孔隙水的化学反应过程,及其离子交换与反应物的关系。对早期成岩作用过程与机理进行了探讨.提出河口地区以泥质为主和以沙质为主的两种早期成岩作用基本类型及其特征与标志。为研究古代类似沉积物早期成岩作用的识别提供了依据。     

开江-梁平地区上二叠统长兴组储层岩石学特征

以露头剖面测量、单井岩芯观察和室内薄片鉴定为依据,结合不同结构和成因类型白云岩(或白云石)的n(Mg)/n(Ca)比值,有序度和C、O、Sr同位素特征,对开江-梁平地区长兴组碳酸盐岩储层成岩作用与孔隙发育和演化规律进行了综合研究,确定研究区对储层破坏最大的成岩作用是胶结、压实-压溶、新生变形和次生矿物的充填等作用,而对储层贡献最大的建设性成岩作用主要为埋藏白云岩化、重结晶、溶蚀和破裂等作用,有效地改善了储层的孔渗性,形成以粒间溶孔、粒内溶孔、铸模孔、晶间溶孔、超大溶孔、溶洞和溶裂缝等多种次生孔隙类型组合的礁、滩相储层基本特征。在再造成岩序列和恢复孔隙演化历史的基础上,对储层发育与孔隙演化的关系可得出如下几点结论:①有利的礁、滩相带决定了储层发育的位置和空间展布规律;②礁、滩复合体的多期次埋藏白云岩化是形成储层的基础;③破裂作用和溶蚀作用是改造储层和提高储层质量的关键。     

Diatoms,spatial distribution and physicochemical characteristics of the Wular lake sediments,Kashmir valley,Jammu and Kashmir

Lakes and wetlands are dynamic geomorphic units of a landscape that hold geochemical signatures of sediment provenance and paleo-environmental shifts and are major sinks for organic matter accumulation. The source of organic matter is diverse in lake sediments and varies widely with the type and size of the lake and hence it is important to understand the source of organic carbon (terrestrial or in situ) in lake systems in order to monitor the health of the lake. Wular lake, located in north Kashmir, is one of the largest fresh water lake in India, situated at an average elevation of 1580m ASL. The lake is fed by a number of watersheds that bring a diverse type of sediments and organic matter and thus deposit them into the Wular lake basin. In order to understand sediment distribution pattern, content and source of organic matter, sediment provenance and the persisting environment in the Wular lake, 32 lake floor sediment samples covering the entire lake were collected and analyzed for organic element analysis, CaCO3, organic matter, sediment texture and diatom analysis. The results indicated that sediments in the lake are dominated by silt and silty clay. The organic carbon in the lake ranged from 0.83%-4.52% and nitrogen varied from 0.06%-0.5%. The Carbon to Nitrogen (C/N) ratios (9.04 to 22.03) indicate a mixed source of organic carbon but dominated by in situ lake sources from the vascular and lake biota accumulation. The diatom analysis revealed the occurrence of a diverse type of species along the sampling sites present within the lake. The wide distribution of the diatom species such as Cymbella, Cyclotella and Tabularia etc. in the lake indicate high organic pollution and alkaline fresh water environment prevailing in the lake.     

Diagenesis of early Permian high‐latitude limestones,Lower Parmeener Supergroup,Tasmania

The Darlington (Sakmarian) and Berriedale (Artinskian) Limestones are neritic deposits that accumulated in high‐latitude environments along the south‐eastern margin of Pangea in what is now Tasmania. These rocks underwent a series of diagenetic processes that began in the marine palaeoenvironment, continued during rapid burial and were profoundly modified by alteration associated with the intrusion of Mesozoic igneous rocks. Marine diagenesis was important but contradictory; although dissolution took place, there was also coeval precipitation of fibrous calcite cement, phosphate and glauconite, as well as calcitization of aragonite shells. These processes are interpreted as having been promoted by mixing of shelf and upwelling deep ocean waters and enabled by microbial degradation of organic matter. In contrast to warm‐water carbonates where meteoric diagenesis is important, the Darlington and Berriedale Limestones were largely unaffected by meteoric diagenesis. Only minor dissolution and local cementation took place in this diagenetic environment, although mechanical compaction was ubiquitous. Correlation with burial history curves indicates that chemical compaction became important as burial depths exceeded 150 m, promoting precipitation of extensive ferroan calcite. This effect resulted from burial by rapidly deposited, overlying, thick, late Permian and Triassic terrestrial sediments. This diagenetic pathway was, however, complicated by the subsequent intrusion of massive Mesozoic diabases and associated silicifying diagenetic fluids. Finally, fractures most probably connected with Cretaceous uplift were filled with late‐stage non‐ferroan calcite cement. This study suggests that both carbonate dissolution and precipitation occur in high‐latitude marine palaeoenvironments and, therefore, the cold‐water diagenetic realm is not always destructive in terms of diagenesis. Furthermore, it appears that for the early Permian of southern Pangea at least, there was no real difference in the diagenetic pathways taken by cool‐water and cold‐water carbonates.     

河西猪野泽沉积物有机地化指标之间的关系及古环境意义

探讨了河西走廊石羊河终端湖泊--猪野泽晚冰期以来沉积物总有机碳(TOC)、碳氮比(C/N)、有机碳同位素(δ13C)三种有机地球化学指标之间的关系及古环境意义.结果表明:猪野泽QTH01剖面沉积物TOC指标主要代表湖泊及流域的植被初级生产力;C/N值受到周围区域表土C/N值和沉积物成岩过程的双重影响,总体较低,并不能直...     

Secondary dispersion of trace elements in bottom sediments of the High Dam Lake,South Egypt and North Sudan

Thirty-four chemical elements, pH, total nitrogen, and total organic carbon were determined in 49 bottom sediment samples from the whole High Dam Lake in order to improve our understanding of geochemical characteristics of these sediments and geochemical patterns of trace elements and related feeding sources. The present study revealed that the lake were clearly discriminated into three portions in accordance with the sediment geochemistry and geographic position. Likewise, the analyzed elements in the entire lake sediments were classified into six geochemical association patterns that are indicative of the contributing geogenic and anthropogenic sources. As has been noted here, mineralogy, anthropogenic inputs, pH, and organic matter had significant roles in controlling the behavior, concentration, dispersion, and geochemical patterns of the trace elements in the lake sediments. Consequently, the elevated concentration of Bi, Cd, Co, Cr, Cu, Fe, Ga, Mn, Mo, Ni, Sc, V, Y, and Zn posed moderate contamination level in the sediments. At the same time, the enhancement of Ag, Se, and Te levels caused contamination up to very high levels. Admittedly, the contamination levels were generated by natural and human activities that are coming from the Nile basin countries. Despite progressive deterioration of these sediments, they still have economic applications.     

Deep‐burial alteration of early‐diagenetic carbonate concretions formed in Palaeozoic deep‐marine greywackes and mudstones (Bardo Unit,Sudetes Mountains,Poland)

Carbonate concretions formed in bathyal and deeper settings have been studied less frequently than those formed in shallow‐marine deposits. Similarly, concretions affected by catagenetic conditions have rarely been reported. Calcite concretions in deep‐marine mudstones and greywackes of the Bardo Unit (Sudetes Mountains, Poland) formed during early diagenesis and were buried to significant depths. Petrographic and geochemical (elemental and stable C and O isotopic) analyses document their formation close to the sediment–water interface, prior to mechanical compaction within the sulphate reduction zone and their later burial below the oil window. Although the concretions were fully formed during early diagenesis, the effects of increased temperature and interaction with late‐diagenetic interstitial fluids can be discerned. During maximum burial, the concretions underwent thorough recrystallization that caused alteration of fabric and elemental and O isotope composition. The initial finely crystalline cement was replaced by more coarsely crystalline, sheaf‐like, poikilotopic calcite in the concretions. These large calcite crystals engulf and partially replace unstable detrital constituents. The extremely low δ¹⁸O values (down to ?21·2‰ Vienna Pee Dee Belemnite) in the concretions are the result of the increased temperature in combination with alteration of volcanic glass, both causing a significant ¹⁸O‐depletion of bicarbonate dissolved in the interstitial fluids. Recrystallization led to uniform O isotope ratios in the concretions, but did not affect the C isotope signature. The δ¹³C values of the late‐diagenetic cements precipitated in the greywacke and in cracks cutting through concretions imply crystallization in the catagenetic zone and decarboxylation as a source of the bicarbonate. These late‐diagenetic processes took place in a supposedly overpressured setting, as suggested by clastic dykes and hydrofractures that cut through both concretions and host rock. All of these features show how the effects of early and late diagenesis can be distinguished in such rocks.     

中国淡水湖泊沉积物地球化学背景与环境质量基准建立的思考

利用中国150余个淡水湖泊表层沉积物和深层沉积物的地球化学数据,采用中位数绝对中位差的方法统计获得了中国主要淡水湖泊沉积物的As、Cd、Cr、Cu、Hg、Ni、Pb、Zn、TN、TP、TOC及pH的地球化学背景值,为我国湖泊沉积物环境质量基准的建立和湖泊生态环境质量监管提供了重要依据。统计结果表明,我国表层湖泊沉积物中的As、Cd、Hg等重金属元素以及TP、TOC等发生了显著富集,显示出强烈的人类活动对湖泊沉积物环境质量的改变。在探讨国外环境质量基准值对我国淡水湖泊沉积物环境质量评价适应性的基础上,提出直接采用国外相关标准还不能客观评价我国湖泊沉积物的污染程度和生态风险,应充分考虑我国湖泊沉积物的地球化学背景,建立适应我国情况的沉积物环境质量基准。     

Diagenesis of vascular plant organic matter components during burial in lake sediments

Diagenetic changes are difficult to distinguish from variations in sources of organic matter to sediments. Organic geochemical comparisons of samples of wood, bark, and needles from a white spruce (Picea glauca) living today and one buried for 10,000 years in lake sediments have been used to identify the effects of diagenesis on vascular plant matter. Important biogeochemical changes are evident in the aged spruce components, even though the cellular structures of the samples are well preserved. Concentrations of total fatty acids dramatically diminish; unsaturated and shorter chainlength components are preferentially lost from the molecular distributions. Concentrations of total alcohols are similar in the modern and 10,000-year-old wood and bark but markedly lowered in the aged needles. Hydrocarbon concentrations and distributions show little diagenetic change in the 10,000-year-old plant materials. Cellulose components in the wood decrease relative to lignin components, although both types of materials remain in high concentration in comparison to other organic components. Aromatization of abietic acid proceeds more rapidly in buried spruce wood than in bark; retene is the dominant polyaromatic hydrocarbon in the aged wood. In contrast to the variety of changes evident in molecular compositions, neither ¹³C values nor C/N ratios differ significantly in the bulk organic matter of modern and aged spruce components.     

Holocene variations in the Asian monsoon inferred from the geochemistry of lake sediments in central Tibet

We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of % carbonate, % organic carbon, C/N and δ¹³C of bulk organic matter, δ¹³C and δ¹⁸O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (∼7500 cal yr B.P.). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr B.P. and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ∼4000 yr. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period.     

Early diagenesis in anaerobic lake sediments: chemical equilibria in interstitial waters

As part of a study of early diagenesis in anaerobic lake sediments the major ions and P, Fe, Mn, pS, p? and pH were measured in interstitial waters of cores from eutrophic Greifensee in north-central Switzerland. A guide for the interpretation of such data is thermodynamic equilibrium between the dissolved species and mineral phases. The results show that CaCO₃, which precipitates from the lake water, dissolves rapidly in the CO₂-rich interstitial waters resulting in equilibrium with calcite in the top few centimeters of sediment. Siderite, on the other hand, is nearly ten-fold supersaturated 20–30 cm below the surface. Iron monosulfides are metastable in the surface 10–20 cm of sediment and pS and p? results indicate equilibrium between polysulfides and orthorhombic sulfur. Vivianite appears to be limiting the solubility of phosphate below a few centimeters of sediment.The above processes are highly interactive, especially in the case of ferrous iron, with one result being a two order of magnitude decrease in dissolved phosphate in a depth interval of 20cm in the interstitial waters.